100 HOM' CROPS FEED. 



wliich compose the atmosphere are uiiiforinly mixed to* 

 gether without regard to their specific gravity, is but one 

 result of a law of nature which we shall immediately 

 notice. 



Diffusion of Gases, — Whenever two or more gases are 

 brought into contact in a confined space, they instantly 

 begin to intermingle, and continue so to do until, in a 

 longer or shorter time, they are both equally diffused 

 throughout the room they occuj^y. If two bottles, one 

 filled with carbonic acid, the other with hydrogen, be con- 

 nected by a tube no wider than a straw, and be placed so 

 that the heavy carbonic acid is below the fifteen times 

 lighter hydrogen, we sliall find, after the lapse of a few 

 hours, that the two gases have mingled somewhat, and in 

 a few days they will be in a state of uniform mixture. On 

 closer study of this phenomenon it has been discovered 

 that gases diffuse with a rapidity proportioned to their 

 lightness, the relative diffusibility being nearly in the in- 

 verse ratio of the square roots of their specific gravities. 

 By interposing a poious diaphragm between two gases of 

 different densities, we may visibly exhibit the fact of their 

 ready and unequal diffusion. For this purpose the dia- 

 phragm must offer a partial resistance to the movement 

 of the gases. Since the lighter gas passes more rapidly 

 into the denser than flie reverse, the space on one side of 

 the membrane will be overfilled, while that on the other 

 side will be partially emptied of gas. 



In the accompanying figure is represented a long glass 

 tube, b, widened above into a funnel, and having cemented 

 upon this an inverted cylindrical cup of imglazed porce- 

 lain, a. The funnel rests in a round aperture inade in the 

 horizontal arm of the support, wliile the tube below dips 

 beneath the surface of some water contained in the wine- 

 glass. The porous cup, funnel, and tube, being occujiied 

 with common air, a glass bell, c, is filled with hydrogen 

 gas and placed over the cap, as shown in the figure. In- 



